Over 6,200,000 fractures of the skeleton occur in the United States each year, with almost 10% complicated by disrupted patterns of bone healing (Einhorn, T. A., J. Bone Jt. Surg. 77-A:940–956, 1995). Even with a majority of fractures healing appropriately, over 30,000,000 days each year are lost because of disability or confinement of patients, leading to a tremendous loss of productivity and income. Given the great potential of both tissue and genetic engineering and gene therapy, it is anticipated that exogenous acceleration of fracture healing could increase the overall numbers of fractures that heal successfully, as well as reduce the number of patient days lost due to incapacity.
A number of biochemical and biophysical interventions have been devised with the goal of accelerating the healing of skeletal fractures. The biological strategies include biomimetically inspired skeletal graft substitutes (hydroxyapatite, calcium carbonate), purified or recombinant molecules with chondrogenic and osteogenic attributes (i.e., growth factors), gene therapy and stem cell reservoirs introduced by biodegradable matrices (Hannouche, D., et al., J. Bone Jt. Surg 83, 157–164, 2001). The biophysical arsenal includes low intensity ultrasound (Hadjiargyrou, M., et al., Clin. Orthop 355, S216–S229, 1998; Rubin, C., et al., J. Bone Jt. Surg. 83-A, 259–270, 2001), mechanical stimuli (Kenwright, J. & Gardner, T., Clin. Orthop. 355, S179–S190, 1998), and electromagnetic fields (Otter, M. W., et al., Clin. Orthop. 355, S90–S104, 1998). While the basic science foundation supporting these modalities is strong, the clinical results have been inconclusive. Therefore, it becomes reasonable to conclude that the complexity of the healing process is being underestimated and that the healing process cannot ultimately be determined by a singular idealized molecule, material, or stimulus.
The great majority of studies that have examined the molecular basis of healing have focused on the expression of specific genes, with the bulk of these studies concentrating on the regulatory role of known extracellular matrix (ECM)1 genes (Jingushi, S., et al., J. Bone Miner. Res. 7:1045–1055, 1992; Sandberg, M. M., et al., Clin. Orthop. Rel. Res. 289:292–312, 1993; Hirakawa, K., et al., J. Bone Miner. Res. 9:1551–1557, 1994; Hiltunen, A., et al., FEBS Lett. 364:171–174, 1995; Yamazaki, M., et al., J. Orthop. Res. 15:757–764, 1997) and growth factor genes and proteins (Linkhart, T. A., et al., Bone (NY) 19, 1S–19S, 1996; Barnes, G. L., et al., J. Bone Miner. Res. 14:1805–1815, 1999). The range of genes evaluated has recently expanded to include other protein families, including intracellular signaling molecules (Zhu, W., et al., J. Bone Miner. Res. 16:535–540, 2001), transcription factors (Sakano, S., et al., J. Bone Miner. Res. 14:1891–1901, 1999; Uusitalo, H., et al., J. Bone Miner. Res. 16:1837–1845, 2001), cytokines (Einhorn, T. A., et al., J. Bone Miner. Res. 10:1272–1281, 1995; Ohta, S., et al., J. Bone Miner. Res. 14:1132–1144, 1999), adhesion molecules (Einhorn, T. A., Clin. Orthop. 355, S7–21, 1998), and enzymes (Matsui, N., et al., Biochem. Biophys. Res. Commun. 229:571–576, 1996; Diwan, A. D., et al., J. Bone Miner. Res. 15:342–351, 2000). While this body of work has helped demonstrate the temporal and spatial roles of specific genes, it has also served to indicate that we have limited knowledge of how extensive the transcriptional control of the repair process may be or identify specific genes and processes that are the critical regulators of successful bone healing.
Given the biological complexity of fracture healing, a process morphologically characterized by inflammation, chondrogenesis, and osteogenesis, it is reasonable to hypothesize that it is regulated by a very large number of transcriptional events (Hadjiargyrou, M., et al., J. Bone Miner. Res. 15:1014–1023, 2000; Hadjiargyrou, M., et al., Bone (NY) 9:149–154, 2000). This hypothesis is supported by the marked similarities between the repair process of bone and embryonic development of the skeleton, marked by key cellular events (migration, adhesion, proliferation, and differentiation), all of which require the tightly orchestrated activity of thousands of proteins whose expression patterns rely on both extracellular and intracellular signals. However, little is known in the prior art about the actual processes or the genes involved.
What is need are novel compositions and methods for use in the diagnosis, screening and therapeutic intervention (e.g., gene therapies) in relation to bone healing and regeneration.